U.S. patent number 7,080,521 [Application Number 10/930,635] was granted by the patent office on 2006-07-25 for mobile refrigeration system and control.
This patent grant is currently assigned to Thermo King Corporation. Invention is credited to Gary Connolly, Steve Helgeson, Doug Lenz, Bradley M. Ludwig, Brian Meagher, Darrell Storlie.
United States Patent |
7,080,521 |
Ludwig , et al. |
July 25, 2006 |
Mobile refrigeration system and control
Abstract
A mobile refrigeration system that includes an engine that is
operable at a first speed greater than zero and a second speed
greater than zero. A compressor is operable in response to the
engine at a first speed and a second speed. The system also
includes an evaporator, a first temperature sensor positioned to
measure a first temperature, and a second temperature sensor
positioned to measure a second temperature. A controller is
operable to transition the engine between the first speed and the
second speed in response to the first temperature exceeding a first
predetermined value and the second temperature falling below a
second predetermined value.
Inventors: |
Ludwig; Bradley M. (Minnetonka,
MN), Lenz; Doug (Prior Lake, MN), Storlie; Darrell
(Clarks Grove, MN), Helgeson; Steve (Lakeville, MN),
Meagher; Brian (Prior Lake, MN), Connolly; Gary (Galway,
IE) |
Assignee: |
Thermo King Corporation
(Minneapolis, MN)
|
Family
ID: |
35941089 |
Appl.
No.: |
10/930,635 |
Filed: |
August 31, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060042296 A1 |
Mar 2, 2006 |
|
Current U.S.
Class: |
62/208; 62/323.4;
62/239; 62/228.4 |
Current CPC
Class: |
F25B
49/025 (20130101); F25D 29/00 (20130101); F25D
11/003 (20130101); F25B 2600/111 (20130101); F25B
2700/21173 (20130101); F25B 2600/0252 (20130101); F25B
2700/11 (20130101); F25B 2700/21172 (20130101); F25B
2327/001 (20130101); Y02B 40/00 (20130101); F25B
41/22 (20210101); F25B 2600/112 (20130101) |
Current International
Class: |
B60H
1/32 (20060101) |
Field of
Search: |
;62/208-209,228.4-228.5,239,323.4 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Thermo King Corporation brochure on Model SB-210 Temperature
Control Unit (TK 52031-2-PL) Dated Oct. 2003. cited by other .
Thermo King Corporation brochure on Model SB-310 Temperature
Control Unit (TK 52032-2-PL) Dated Oct. 2003. cited by
other.
|
Primary Examiner: Tapolcai; William E
Attorney, Agent or Firm: Michael Best & Friedrich,
LLP
Claims
What is claimed is:
1. A mobile refrigeration system comprising: an engine operable at
a first speed greater than zero and a second speed greater than
zero; a compressor operable in response to the engine at a first
speed and a second speed; an evaporator; a first temperature sensor
positioned to measure a first temperature; a second temperature
sensor positioned to measure a second temperature; and a controller
operable to transition the engine between the first speed and the
second speed in response to the first temperature exceeding a first
predetermined value and the second temperature falling below a
second predetermined value.
2. The mobile refrigeration system of claim 1, wherein the engine
is a diesel engine.
3. The mobile refrigeration system of claim 1, further comprising
an evaporator fan and a condenser fan, each operable at a first
speed and a second speed in response to the speed of the
engine.
4. The mobile refrigeration system of claim 1, wherein the first
temperature sensor is positioned to measure a return air
temperature and the second temperature sensor is positioned to
measure an evaporator discharge air temperature.
5. The mobile refrigeration system of claim 1, wherein the first
predetermined value is offset a user selectable amount from a first
user selectable value.
6. The mobile refrigeration system of claim 5, wherein the second
predetermined value is the difference between a user selected
deltaT value and the first user selectable value.
7. The mobile refrigeration system of claim 6, wherein the deltaT
value is between about 1 degree and 6 degrees Fahrenheit.
8. The mobile refrigeration system of claim 5, wherein the second
predetermined value is the difference between a user selected
deltaT value and a fixed temperature.
9. The mobile refrigeration system of claim 8, wherein the fixed
temperature is about 35 degrees Fahrenheit and the deltaT value is
between about 1 degree and 6 degrees Fahrenheit.
10. The mobile refrigeration system of claim 1, wherein the
controller includes a microprocessor-based controller.
11. The mobile refrigeration system of claim 1, further comprising
a valve operable to vary a flow of refrigerant from the
compressor.
12. The mobile refrigeration system of claim 11, wherein the valve
includes a suction line throttle valve.
13. The mobile refrigeration system of claim 11, wherein the valve
includes an unloader valve.
14. The mobile refrigeration system of claim 11, further comprising
a defrost member.
15. The mobile refrigeration system of claim 14, wherein the
defrost member includes a defrost heat exchanger.
16. The mobile refrigeration system of claim 15, wherein the
controller initiates a defrost in response to the measured first
temperature, the measured second temperature, and a measured valve
position.
17. The mobile refrigeration system of claim 1, further comprising
a timer operable to measure a duration, wherein the engine
transitions between the first speed and the second speed in
response to the first temperature exceeding the first predetermined
value and the second temperature falling below the second
predetermined value for a duration measured by the timer that is
greater than a predetermined duration.
18. The mobile refrigeration system of claim 1, wherein the
controller is operable to transition the engine from the second
speed to the first speed in response to one of the first measured
temperature falling below a switch point value and a summation of
an integral error exceeding a predetermined maximum integral
error.
19. The mobile refrigeration system of claim 18, wherein the
integral error is inversely proportional to a temperature error.
Description
BACKGROUND
The present invention relates to a mobile refrigeration system.
More particularly, the present invention relates to an
engine-driven mobile refrigeration system that includes an
automatic control system.
Mobile refrigeration systems are often used to chill or cool a
storage area within a mobile container, such as a truck trailer.
Often, perishable items, such as fruits and vegetables, are
transported using these systems. The shelf life and appearance of
these products is greatly affected by the temperature at which they
are maintained during shipping. For example, too low a temperature
can cause freezing, which damages some of the products being
shipped. Too high of a temperature may cause spoilage or rotting of
some products that are shipped.
New trailers are getting larger and include less insulation. In
addition, the insulation in old trailers degrades over time.
Furthermore, trailers are commonly used across a wide ambient
temperature range, thus requiring precise temperature control
across a much wider capacity range. As such, current transport
systems have difficulty maintain the temperature of the products
within a narrow range without excess engine operation. The excess
engine operation results in additional engine and other component
wear, additional maintenance, and additional fuel costs.
SUMMARY
The present invention provides a mobile refrigeration system that
includes an engine that is operable at a first speed greater than
zero and a second speed greater than zero. A compressor is operable
in response to the engine at a first speed and a second speed. The
system also includes an evaporator, a first temperature sensor
positioned to measure a first temperature, and a second temperature
sensor positioned to measure a second temperature. A controller is
operable to transition the engine between the first speed and the
second speed in response to the first temperature exceeding a first
predetermined value and the second temperature falling below a
second predetermined value.
The invention also provides a mobile refrigeration system that
includes an engine that is operable at a first speed and a second
speed. A compressor is operable in response to operation of the
engine to produce a flow of compressed refrigerant. A valve is
associated with the compressor and is movable between a first
position and a second position to vary the flow of compressed
refrigerant. A fan is operable in response to operation of the
engine to produce a flow of air. A first temperature sensor is
positioned to measure a first temperature and a second temperature
sensor is positioned to measure a second temperature. A timer is
operable to time a duration and a microprocessor-based controller
is operable to vary the valve position to maintain the first
temperature at about a user set point. The controller is also
operable to transition the engine between the first speed and the
second speed in response to a measured first temperature in excess
of a first predetermined value and the second measured temperature
less than a second predetermined value and a timed duration greater
than a predetermined time.
The invention also provides a method of controlling a mobile
refrigeration unit. The method includes operating an engine at a
first speed and operating a compressor at a first speed in response
to engine operation to produce a flow of compressed refrigerant.
The method further includes measuring a first temperature and
moving a valve in response to the measured first temperature to
maintain the first temperature at about a first user defined
temperature. The method also includes measuring a second
temperature and transitioning the engine to a second speed greater
than the first speed in response to the measured second
temperature. The method further includes moving the valve in
response to the second temperature to maintain the second
temperature at about a second user defined temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
The description particularly refers to the accompanying figures in
which:
FIG. 1 is a schematic illustration of a mobile refrigeration
compartment including a refrigeration system;
FIG. 2 is a schematic illustration of a refrigeration cycle;
FIG. 3 is a simplified flowchart illustrating a portion of the
operation of the refrigeration system of FIG. 1;
FIG. 4 is a flowchart illustrating a portion of the operation of
the refrigeration system of FIG. 1; and
FIG. 5 is a ladder diagram illustrating various temperature
relationships.
Before any embodiments of the invention are explained, it is to be
understood that the invention is not limited in its application to
the details of construction and the arrangements of components set
forth in the following description or illustrated in the following
drawings. The invention is capable of other embodiments and of
being practiced or of being carried out in various ways. Also, it
is to be understood that the phraseology and terminology used
herein is for the purpose of description and should not be regarded
as limiting. The use of "including," "comprising," or "having" and
variations thereof is meant to encompass the items listed
thereafter and equivalence thereof as well as additional items. The
terms "connected," "coupled," and "mounted" and variations thereof
are used broadly and encompass direct and indirect connections,
couplings, and mountings. In addition, the terms "connected" and
"coupled" and variations thereof are not restricted to physical or
mechanical connections or couplings.
DETAILED DESCRIPTION
With reference to FIG. 1, a cargo space 10 such as would be found
within a truck trailer is illustrated. The cargo space 10 includes
a floor 15, a ceiling 20, two side walls 25, a front wall 30, and a
rear wall 35. Generally, the rear wall 35 includes a door that
allows for convenient loading and unloading of the cargo space 10.
In most constructions, the walls 25, 30, 35 the floor 15, and the
ceiling 20 are insulated to make temperature control of the cargo
space 10 more efficient.
A refrigeration system 40 is attached to the outside of the front
wall 30 with other locations being possible. The refrigeration
system 40 draws relatively warm air from within the cargo space 10,
cools the air, and returns the cold air to the cargo space 10. The
front wall 30 of the cargo space 10 includes a return air aperture
45 that provides for the passage of air from the cargo space 10
into the refrigeration system 40. Generally, a bulkhead 50 that may
include an air filter at least partially defines the aperture
45.
Cold air exiting the refrigeration system 40 is generally directed
to an air delivery duct 55 disposed on the ceiling 20 of the cargo
space 10. The air delivery duct 55 distributes the cold air
substantially evenly throughout the cargo space 10 to assure that
the entire cargo space 10 is evenly cooled.
With reference to FIG. 2 the components of the refrigeration system
40 are illustrated. Before describing the system 40, it should be
noted that many components, including valves, sensors, tanks,
manifolds, and the like have been omitted from the diagram for
clarity.
The refrigeration system 40 includes a diesel engine 60 that
functions as the prime mover for the system. In other
constructions, other engines (e.g., gasoline, Stirling, combustion
turbine, hybrid, and the like) may be used as the prime mover. The
refrigeration system 40 also includes a compressor 65 that is
driven by the engine 60 to produce a flow of compressed refrigerant
(e.g., R12, freon, ammonia, etc.). The engine 60 drives the
compressor 65 such that the compressor 65 operates at a speed that
is proportional to the speed of the engine 60. In many
constructions, a belt or chain drive 70 is employed to couple the
engine 60 and the compressor 65. However, other constructions may
employ a direct drive, a gear drive, or another type of coupling or
transmission. Many types of compressors can be employed including,
but not limited to, screw compressors, reciprocating compressors,
and scroll compressors.
The compressor 65 draws refrigerant from a suction line 75 and
compresses the refrigerant to produce a flow of compressed
refrigerant. The compressed refrigerant flows to a condenser 80
where excess heat is removed. The condenser 80 includes a heat
exchanger that transfers heat energy from the compressed
refrigerant to an air stream 85. A condenser fan 90, driven by the
engine 60, moves the air stream 85 through the condenser 80 to
facilitate the efficient removal of heat. As with the compressor
65, preferred constructions employ a belt or chain drive 95 between
the condenser fan 90 and the engine 60 that assures that the
condenser fan 90 operates at a speed that is proportional to the
speed of the engine 60. In other constructions, different coupling
means such as gears, direct drives, or other types of transmissions
may be employed to allow the engine 60 to drive the condenser fan
90.
As the flow of compressed refrigerant passes through the condenser
80, the refrigerant generally condenses to a liquid state. The
high-pressure liquid next flows to an expansion valve 100 where the
pressure is reduced, thereby also reducing the temperature of the
refrigerant. The cold refrigerant then flows into an evaporator
105.
The evaporator 105 includes a second heat exchanger that transfers
heat energy from a second air stream 110 that is drawn from the
cargo space 10 to the refrigerant. Thus, the evaporator 105 cools
the second air stream 110. As with the condenser 80, the evaporator
105 includes an evaporator fan 115 that is driven by the engine 60.
The evaporator fan 115 moves the second air stream 110 through the
evaporator 105 and back into the cargo space 10 to facilitate the
efficient cooling of the air stream 110. As with the condenser fan
90, preferred constructions employ a belt or chain drive 120
between the evaporator fan 115 and the engine 60 that assures that
the evaporator fan 115 operates at a speed that is proportional to
the speed of the engine 60. In other constructions, different
coupling means such as gears, direct drives, or other types of
transmissions may be employed to allow the engine 60 to drive the
evaporator fan 115.
After the refrigerant leaves the evaporator 105, it returns to the
suction line 75 that feeds the compressor 65, thus completing the
cycle. As one of ordinary skill in the art will realize, many other
components may be employed in the system just described. For
example, multiple compressors 65, evaporators 105, condensers 80,
evaporator fans 115, or condenser fans 90 could be employed in one
system if desired. In addition, storage tanks, reservoirs,
liquid-to-suction heat exchangers, economizers, unloader valves,
and hot-gas bypass valves could be employed at various points
within the system.
With continued reference to FIG. 2, the refrigeration system 40
also includes a suction line throttle valve 125. The suction line
throttle valve 125 moves between a first, or closed position and a
second, or open position. In the closed position, the valve 125
restricts the quantity of refrigerant delivered to the compressor
65 and thus reduces the cooling capacity of the refrigeration
system 40. As the valve 125 moves toward the open position,
additional refrigerant is able to pass through the valve 125 to
increase the cooling capacity of the refrigeration system 40. In
most constructions, the valve 125 is electrically controlled and
actuated. However, other constructions may employ other types of
valves (e.g., mechanically controlled and actuated) if desired.
Other constructions may also employ valves that are positioned
differently than the suction line valve 125 (e.g., unloader valves)
but that still function to control the cooling capacity of the
refrigeration system 40 by varying the flow of refrigerant to or
from the compressor 65.
In some constructions, a third heat exchanger 130 is positioned
adjacent the evaporator 105 or actually intermingles with the
evaporator 105. The third heat exchanger 130 receives a flow of
heated fluid that can be used to defrost the evaporator 105. For
example, one construction of the refrigeration system 40 directs
engine coolant from the engine 60 through the third heat exchanger
130 to periodically defrost the evaporator 105.
The system 40 includes a controller 135 that is interconnected with
the engine 60 and a plurality of sensors to monitor and control the
refrigeration system 40. In preferred constructions, a
microprocessor-based controller is employed. However, other
constructions may employ an analog electric control system such as
a series of switches and relays or another controller (e.g.,
mechanical control system, PLC based system, and the like) as
desired. The use of the microprocessor-based controller allows for
greater flexibility and more accurate control than what could be
achieved using other types of controllers.
Among the many sensors that may be employed, the refrigeration
system generally includes a return air sensor 140 that measures the
temperature of the air returning from the cargo space 10.
Generally, the return air temperature provides a good indication of
the actual temperature of the product being shipped within the
cargo space 10. Another sensor typically employed is a discharge
air temperature sensor 145. The discharge air temperature sensor
145 measures the temperature of the air leaving the evaporator 105.
Generally, this is the lowest air temperature within the system 40.
In many systems 40, redundant sensors 140, 145 are provided such
that the failure of one or more sensors does not disable the entire
refrigeration system 40.
In most constructions, the refrigeration system 40 also includes a
valve position sensor 150. The valve position sensor 150 measures
the actual position of the valve 125 and returns a signal to the
controller 135 that is representative of the actual valve position.
While many different types of sensors or feedback are possible,
LVDTs (linear variable differential transformers) and RVDTs
(rotational variable differential transformers) are preferred. In
other constructions, a stepper motor is used to drive the valve 125
and the position of the stepper motor is monitored using software,
thus eliminating the need for position feedback.
The refrigeration system 40 described herein is capable of
operating in several modes depending on the operating conditions of
the system 40 as well as ambient conditions outside of the cargo
space 10. In addition, the controller 135 is able to automatically
transition the system 40 between the various modes.
One mode of operation illustrated in FIG. 3 is return air control
with modulation. In this mode, the controller 135 monitors the
return air temperature (RAT) (shown in block 155) and manipulates
the suction line throttle valve 125 in an effort to maintain the
measured return air temperature at or near a user defined return
air set point value T I. Generally, the user defined return air set
point temperature T1 is between about 15 degrees and 90 degrees
Fahrenheit. Of course, colder or warmer temperatures could be
selected if desired. As the throttle valve 125 opens, more
refrigerant is drawn into the compressor 65, thereby increasing the
cooling capacity of the refrigeration system 40. However, the air
flow through the evaporator 105 remains substantially constant as
the evaporator fan 115 moves at a constant speed. Thus, the air
exiting the evaporator 105 is cooler. This air temperature is
measured (at block 155) as the discharge air temperature (DAT).
To further improve the control of the temperature within the cargo
space 10, a lower limit is placed on the discharge air temperature
when operating in return air control. This limit is generally
referred to as the discharge air floor limit T2. The discharge air
floor limit T2 is generally determined by subtracting a user input
deltaT (.DELTA.T) value from the user defined return air set point
value T1. For example, if a user selects a return air set point T1
of 40 degrees Fahrenheit and further selects a deltaT value of 5
degrees Fahrenheit, the discharge air floor limit T2 would be 35
degrees Fahrenheit. In most constructions, a deltaT value between
about 1 degree and 6 degrees Fahrenheit is preferred. However,
other constructions may employ larger or smaller deltaT values.
If, during return air control operation, the discharge air
temperature falls to the floor limit T2, the controller 135
automatically transitions the system 40 to discharge air
temperature control (DAT Control) shown in block 160. When in
discharge air temperature control, the controller 135 manipulates
the suction line throttle valve 125 in an effort to maintain the
discharge air temperature at the floor limit T2.
When controlling based on discharge air temperature, it is possible
for the return air temperature, and the cargo temperature to
continue to rise above the return air setpoint T1 due to many
factors (e.g., high ambient temperature, warm product, product
respiration, air infiltration, insulation degradation, evaporator
airflow restrictions, and the like). The controller 135 monitors
the return air temperature and compares this temperature to a
maximum temperature set point T3. Generally, the maximum
temperature set point T3 is simply an offset 161 from the return
air set point temperature T1. For example, a particular load may
have a return air set point T1 of 40 degrees Fahrenheit and an
offset of 5 degrees Fahrenheit. For this load, the maximum
temperature set point T3 would be 45 degrees Fahrenheit. If the
return air temperature exceeds the maximum temperature set point T3
for a predetermined length of time (e.g., 30 minutes) as measured
by a timer 163 or the controller 135, the system 40 automatically
transitions to high-speed modulation (shown in block 165). In many
constructions, the timer is built into software, thus allowing the
controller to perform the function of the timer.
In high-speed modulation, the engine speed is increased. During
normal operation the engine 60 operates at a first speed. The first
speed provides enough power, airflow, and sufficient temperature
control to operate the refrigeration system 40 under normal load
conditions. However, under some load conditions additional power
and airflow is required. Thus, the engine 60 is able to operate at
a second speed that is higher than the first speed. At the second
speed, the evaporator fan 115 and condenser fan 90 also operate at
a higher speed. As such, both fans 90, 115 are able to push
additional air through the respective heat exchangers 80, 105.
Similarly, the compressor 65 operates at a higher speed, thereby
enabling the compressor 65 to deliver a greater quantity of
refrigerant if necessary.
During high-speed modulation, the controller 135 continues to
manipulate the suction line throttle valve 125 to maintain the
discharge air temperature at the floor limit T2. However, because
additional air is moving through the evaporator 105, the system 40
is able to maintain a substantially constant cooling capacity,
while reducing the temperature differential between the discharge
air temperature and the return air temperature. The reduction in
the temperature difference between the discharge air and the return
air is a result of the additional mass flow of air exiting the
evaporator 105 at the floor limit temperature T2, as compared to
the mass flow when the engine 60 is operating at low speed. This
additional air flow has the effect of reducing the return air
temperature.
The system 40 includes two conditions that facilitate the return to
low-speed modulation from high-speed modulation. If either of these
conditions is met, the system 40 transitions back to low-speed
operation. The first condition occurs when the return air
temperature reaches a switch point T4 that is equal to the return
air temperature set point T1 plus an offset 166 (see block 170).
Generally, an offset 166 of between about 1 and 10 degrees
Fahrenheit is employed with larger or smaller offsets being
possible. For example, if the return air set point T1 is set at 40
degrees Fahrenheit and an offset 166 of 5 degrees Fahrenheit is
employed, the switch point T4 would equal 45 degrees
Fahrenheit.
It should be noted that the maximum temperature set point T3 is
generally offset a fixed amount 167 from the switch point T4. In
most constructions, a 2-degree Fahrenheit offset is employed with
larger or smaller offsets being possible. The 2-degree offset
reduces the likelihood of sudden transitions between high and low
speed in response to minor temperature fluctuations. The
relationships between these various temperatures are best
illustrated in FIG. 5.
The second condition is based on an integral error that accumulates
within the controller (block 175). When the integral error reaches
a maximum integral error value, the system transitions into
low-speed modulation. The integral error accumulates based on the
temperature difference between the measured return air temperature
and a predetermined value (e.g., the return air temperature set
point T1 plus an offset, such as 2 degrees Fahrenheit). However,
unlike a typical integral error, the integral error accumulates
more slowly the greater the temperature error. Thus, a condition
that maintains a high temperature error (e.g., 10 degrees
Fahrenheit) will take longer to reach the maximum integral error
than would a condition that maintains a small temperature error
(e.g., 2 degrees Fahrenheit). Thus, the integral error will allow
the system 40 to operate at high-speed for a longer period of time
if the temperature error is large, but will transition the system
40 back to low speed more quickly for small temperature
differences. For example, a simple refrigeration system may sum the
inverse of the actual error to calculate an integral error. In this
example, a constant error of 2 degrees Fahrenheit would produce an
error of 2 degree-minutes, per minute that the error is maintained.
The inverse of this value would produce an integral error of 0.5
that would increase by 0.5 each minute. The same system, operating
with a 10-degree temperature error would produce an integral error
of 0.1 that would increase by 0.1 each minute. Thus, in this
example it would take five times longer to reach a maximum integral
error value with a 10 degree error than it does with a 2 degree
error.
The integral error assures that the system 40 will eventually
transition back to low speed operation no matter the temperatures
being measured. This reduces the likelihood that the system 40 will
operate at high speed for a long period of time when low-speed
operation would be capable of handling the cooling load.
Freeze protection, a portion of which is illustrated in FIG. 4, is
yet another mode of operation of the refrigeration system 40. When
operating in freeze protection, the floor limit T2 is calculated as
an offset from a base level of 35 degrees Fahrenheit (block 180),
rather than as an offset from the return air set point temperature
T1 (block 185). Thus, the user input deltaT value is subtracted
from 35 degrees Fahrenheit when operating in freeze protection
mode. This mode is particularly well suited for use when the cargo
space 10 contains high-temperature set point goods. For example, if
the return air temperature set point T1 is 45 degrees Fahrenheit
and the delta T value is 3 degrees, the floor limit would be 42
degrees Fahrenheit without using freeze protection. With freeze
protection, the floor limit would be 32 degrees Fahrenheit (i.e.,
35 degrees-3 degrees). The lower floor limit T2 in freeze
protection mode allows the system 40 to remain in low-speed
modulation during operating conditions that would otherwise require
high-speed modulation. The reduced high-speed operation saves
engine fuel and reduces engine wear.
It should be noted that the fixed value of 35 degrees Fahrenheit
used in freeze protection could vary from system to system. As
such, the invention should not be limited to a fixed value of 35
degrees Fahrenheit.
During operation of the refrigeration system 40, cold refrigerant
flowing within the evaporator 105 will cool the evaporator 105. If
the evaporator 105 cools below about 32 degrees Fahrenheit, water
vapor within the air stream 110 will condense and freeze onto the
evaporator 105. As this process continues, the air flow paths
through the evaporator 105 will shrink due to the expanding
quantity of ice. The reduced air flow through the evaporator 105
reduces the cooling capacity of the refrigeration system 40 but
also reduces the discharge air temperature. When operating in
modulation with return air control, the reduced air flow caused by
the ice build-up will result in a rise in return air temperature.
Simultaneously, the reduced air flow paths will produce a drop in
discharge air temperature. At some point, these temperature changes
will transition the system 40 into discharge air control. Once in
discharge air control, the controller 135 will manipulate the
suction line throttle valve 125 to maintain the discharge air
temperature at the floor limit T2. However, as the air flow path
continues to shrink, the discharge air temperature will continue to
drop. The continued drop will cause the controller 135 to move the
suction line throttle valve 125 to a more closed position even as
the return air temperature rises. It is this combination of a
reduction in discharge air temperature coupled with an increase in
return air temperature and the movement of the suction line
throttle valve 125 toward the closed position (block 190 in FIG. 3)
that signals the need for a defrost cycle (block 195). The
controller 135 senses these conditions and initiates the defrost
cycle. Most systems also include an evaporator coil temperature
sensor 200 that can also be used to indicate the need for a defrost
cycle and the end of the defrost cycle. As discussed, there are
various ways to defrost an evaporator 105 (e.g., passing hot engine
coolant or refrigerant through the third heat exchanger 130,
electric heat, etc.), the particular system or method used is not
important to the invention described herein.
After the defrost cycle is complete, the controller 135 transitions
the system 40 to one of the low-speed modulating control modes
(e.g., return air control or discharge air control).
The refrigeration system 40 described is able to maintain the
temperature within the cargo space 10 within a narrow temperature
band that is selected by the user, while also reducing the
operating time of the engine 60 at high speed. The result is a
system that requires less maintenance than prior systems and that
is more fuel-efficient. In addition, the improved temperature
control results in improved quality of the product being
shipped.
It should be noted that many systems may include an electric motor
that serves as a back-up to the engine. In most constructions, a
single-speed electric motor is used. However, other constructions
may employ a two-speed or variable speed motor if desired.
High speed modulation gives the user the ability to control both
the discharge air temperature (i.e., the floor limit) and the
maximum return air temperature at the same time. Prior systems
could only regulate one temperature. Furthermore, the temperature
control can be customized for the particular load by the selection
of various set points and temperature differentials. This allows
the user to balance the temperature requirements with the amount of
high-speed runtime. Thus, a user could select a wider temperature
band to reduce the amount of high-speed operation and the amount of
fuel consumed if desired. The control as described is able to
provide consistent temperature control regardless of the product
hauled, the operating conditions, or the trailer condition.
Although the invention has been described in detail with reference
to certain preferred embodiments, variations and modifications
exist within the scope and spirit of the invention as described and
defined in the following claims.
* * * * *